Six element unit magnification lens

ABSTRACT

An asymmetrical lens of a modified Gauss type adapted for printing at unit magnification, comprising six air-spaced elements surrounding an aperture stop, with front and rear positive biconvex elements, front and rear positive meniscus elements concave toward the aperture stop and front and rear negative elements.

710497 United States .7 111 3,737,215

r De Jager ",7 a. 1 June 5, 1973 [s41 SIX ELEMENT UNIT MAGNIFICATION3,608,452 9/1971 Conrad et al. "350/215 1? LENS FOREIGN PATENTS ORAPPLICATIONS [75] Inventor: Donald De J ager, Rochester, N.Y.

1,120,335 7/l968 Great Britain .350/215 [73] Assignee: Eastman KodakCompany,

Rochester Primary Examiner-John K. Corbin [22] Filed: Apr. 6, 1972Attorney-W. H. J. Kline 211 Appl.No.: 241,572 [5-7] ABSTRACT 152111.5.01 .350 215, 350/210 asymmemca' lens a mmhfied Gauss type adaptedfor printing at unit magnification, comprising [2;] six aipspacedelements surrounding an aperture Stop, e 0 ean: with from and rearpositive biconvex elements from and rear positive meniscus elementsconcave toward [56] Clted the aperture stop and front and rear negativeele- UNITED STATES PATENTS 3,s19,333 7/1970 Takahashi .350/215 4 Claims,6 Drawing Figures PATENTEDJUH 5mm 3.737.215

sumanr 2 PERCENT J PERCENT j FIG. 2 I FIG. 3

PATENTEDJUH Si n 3,737,215

' SHEET 20F 2-- PERCENT ,1

FIG. 5 FIG.6

1 l SIX ELEMENT UNIT MAGNIFICATIONLENS BACKGROUND OF THE INVENTION l.Field of the Invention This invention relates to lenses and inparticular to six element lenses of a modified Gauss type which areadapted for printing at unit magnification.

2. Description of the Prior Art In the printing of microcircuits, theslightest defect in the design or the assembly of the lens may causeundesirable and noticeable effects in the fine line structure of thedeveloped image. One technique used to achieve the high quality requiredin such lenses is to design the lens for use with specific wavelengthsof light in order to minimize certain optical defects. Thus, imagedegradation caused by diffraction of light may be minimized by usingshort wavelength light. Similarly, image degradation caused by chromaticaberration may be minimized by using nearly monochromatic light. Thelight source usually used in the printing of microcircuits is a mercuryvapor lamp, filtered to produce light with a wavelength of 4358 i 100Angstroms. With diffraction and chromatic aberration minimized by theuse of nearly monochromatic short wavelength light, lens performance isdetermined by the corrections for other aberrations.

Many variations of modified Gauss type lenses which are symmetric andare designed for use at unit magnification are known in the prior art.US. Pat. No. 3,348,900 is an example of a six element modified Gausstype lens, which is symmetrical in design and which has its second andthird elements and its fourth and fifth elements cemented together. USPat. No. 3,537,774 is an example of a seven element modified Gauss typelens in which airspaces have been incorporated between the second andthird elements and between the fourth and fifth elements. This lens isalso substantially symmetrical in design since the sixth and seventhelements are cemented. While lateral aberrations are minimized by asubstantially symmetric design as in the cited patents, symmetry doesnot aid in reducing astigmatism or longitudinal spherical aberration.

SUMMARY OF THE INVENTION v An object of the present invention is toprovide six element asymmetric lenses of a modified Gauss type' whichare adapted for use at unit magnification.

Another object of this invention is to provide such lenses which arewell corrected for all aberrations and in particular are well correctedfor astigmatism.

These and other objects are accomplished according to this invention bylenses of a modified Gauss type comprising six airspaced elementsarranged from front to rear in the following order: (1) a front positivebiconvex element, (2) a front positive meniscus element concave awayfrom the front biconvex element, (3) a front negative element, (4) arear negative element, (5) a rear positive meniscus element concavetoward the rear negative element, and (6) a rear positive biconvexelement.

In the preferred embodiments, each lens is adapted for use at unitmagnification and is asymmetric. The asymmetry of each lens may becharacterized by the ratio of the relative thicknesses of the negativeelements T,/T,, by the ratio of the relative air spaces separating thepositive meniscus and negative elements, 8 /8, and by the ratio of therelative air spaces separating thepositive elements 8 /8,. Correction ofastigmatism to within 0.01 percent of the focal length along with goodcorrection of other abberrations has been achieved with lenses whichexhibit asymmetry as characterized by these ratios. In particular, alens is produced which achieves high quality performance withexceptionally good correction of astigmatism when T,/T has a valuebetween 2 and 5, while 8 /8, and S /S each has a value between 1 and9.

BRIEF DESCRIPTION OF THE DRAWINGS In the detailed description 'of thepreferred embodiments of the invention presented below, reference ismade to the accompanying drawings in which:

FIG. 1 is a diagrammatic cross section of a lens according to thisinvention corresponding to Example 1.

FIG. 2 is a graph showing longitudinal spherical abberration in each ofthree wavelengths for the lens of FIG. 1.

FIG. 3 is a graph showing sagittal and tangential astigmatism for thelens of FIG. 1.

FIG. 4 is a diagrammatic cross section of a lens according to thisinvention corresponding to Example 3.

FIG. 5 is a graph showing longitudinal spherical abberation in each ofthree wave lengths for the lens of FIG.- 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT For all purposes of describingor claiming of the invention herein, the term lenswill be used todescribe the complete lens, and not the elements thereof. In FIGS. 1 and4, the elements are numbered from left to right with Arabic numerals. Inthe examples, the elements, the indices of refraction N and Abbe numbersV for the 0.5876 micron Helium line of the spectrum, the radii ofcurvature R, the thicknesses T and the air spaces S are numbered bysubscript to correspond with FIGS. 1 and 4. Radii of curvature havingcenters of curvature to the right of the surface are consideredpositive; those with centers of curvature to the left of the surface areconsidered negative. All parameters are based upon a lens equivalentfocal length of I00 millimeters.

In all embodiments of the invention as illustrated in FIGS. 1 and 4, thelens comprises six air spaced elements surrounding an aperture stop D.Element 1 is a front positive biconvex element. Element 2 is a frontpositive meniscus element concave toward aperture stop D. Element 3 is afront negative element preceding aperture stop D. Element 4 is a rearnegative element following aperture stop D. Element 5 is a rear positivemeniscus element concave toward aperture stop D. Element 6 is a rearpositive biconvex element.

Lenses may be made according to this invention by following thespecifications in the preferred embodiments presented below.

s, 6.714 R, 38.733 2 1.73400 51.3 T,=3.158-

s 1.060 R w 3 1.62588 35.7 T, 3.523

s, 6.643 R 39.814 4 1.69895 30.1 T. 14.517

s, 8.189 R, 112.62 5 1.73400 51.3 T, 3.065

. s 8.474 R" 176.61 6 1.71300 53.9 T, 4.530

The lens design of Example 1 is illustrated in FIG. 1. In this Figure, Ddenotes the aperture stop, P denotes the entrance pupil and P. denotesthe exit pupil, for light traveling from left to right. The distancefrom the object to surface 1 is 155.46 millimeters and this distance isvery nearly equal, at unit magnification, to the distance from surface12 to the image which is 157.06 millimeters.

The entrance pupil P is located near the physical center of the lens.However, the exit pupil P is located near the left end of the lens asmay be seen in FIG. 1. The distance from the object to the entrancepupil is therefore substantially less than the distance from the exitpupil to the image. As a result, even though the object and image areequal in size and very nearly equidistant from the outer surfaces of thelens, the object subtends a somewhat greater angular field with respectto the entrance pupil than does the image with respect to the exitpupil. This is one of the noticeable examples of asymmetry in theproperties of the lens that may be seen by an observer who is unaware ofthe details of the internal construction of the lens.

The internal design of the lens is also asymmetric as may be seen in therelative thicknesses of elements 3 and 4 and the lack of symmetry of theair spaces to the left of aperture stop D with respect to thecorresponding air spaces to the right of aperture stop D. The asymmetricdesign of elements 3 and 4 may be characterized by the ratio of theirthicknesses, TJT The asymmetry of the air spaces on opposite sides ofaperture stop D may be characterized by the ratios of the correspondingair spaces, 5 /8 and by SJS In the lens of Example 1, these thicknessand air space ratios are:

(T /T 4.13 (S /S 1.26 (S /S,) 7.72

The optical aberrations of the lens of Example 1 are shown in FIGS. 2and 3. In FIG. 2, the longitudinal spherical aberration as a percentageof the focal length of the lens is plotted horizontally, versus heightat the entrance pupil which is plotted vertically. The relative apertureindication off/2.75 in FIG. 2 is the equivalent aperture; the effectivef-number is 5.5 in both object erration at the primary wavelength of4358 Angstroms is extremely small, being only 0.011 percent of the focallength. FIG. 3 represents the sagittal and tangential astigmatism as apercentage of the focal length calculated along the chief rays in theprincipal wavelength of 4358 Angstroms according to Coddingtonsformula,"

plotted horizontally, versus the angle at the entrance pupil plottedvertically. Note that the maximum astigmatism, at 3.3, is only 0.006percent of the focal length, an extraordinarily small value.

The lens of Example 1 was designed for use with glasses whose indices ofrefraction exactly match those given in standard optical glass catalogs.In the following Example 2, a different glass type was used in elements2 and 5 and the lens was optimized for use with actual glass sampleswhose measured indices of refraction differed slightly from those givenin optical glass catalogs.

EXAMPLE 2 F mm f/2.75

Radius Thickness or Element N V mm Separation mm S. 6.862 R, 38.752 21.73252 51.7 T,=3.135

' S, 0.983 R5 66 3 1.62552 35.8 T 3.619

S 8.495 R, 113.08 5 1.73252 51.7 T, 3.053

S, 8.202 R 177.80 6 1.71324 54.0 T 4.515

(T /T 3.96 (S /S 1.19 (S /S 8.64

This lens is also well corrected for longitudinal color as well ashaving a maximum astigmatism, at 33, of only 0.007 percent of the focallength.

Example 3, illustrated in FIG. 4, utilizes the same glasses as the lensof Example 1. However, as may be seen from FIG. 4, elements 3 and 4 aremore nearly equal in thickness than in the lens of FIG. 1 and therelative air spaces S and S are more unequal.

EXAMPLE 3 F 100mm f/2.75

Thickness or Element N V Radius Separation mm This lens is also adaptedfor unit magnification with conjugates of 157.16 and 152.43 for objectand image, respectively. The asymmetric nature of the lens of Example 3may be characterized by the following thickness and air space ratios asdefined hereinbefore:

The optical aberrations of the lens of Example 3 are shown in FIGS. 5and 6, which correspond in meaning to FIGS. 2 and 3. The maximumlongitudinal spherical aberration at the primary wavelength of 4358Angstroms is seen to be 0.013 percent of the focal length.

The maximum astigmatism is seen in FIG. 6 to be 0.008

percent of the focal length.

Exceptionally good correction of astigmatism along with good correctionof other aberrations has been achieved in lenses according to thisinvention which are characterized by asymmetry in the thicknesses of thenegative elements and in the relative spacing of the elements within thelens. In particular, correction of astigmatism to within 0.01 percent ofthe focal length along with good correction of other aberrations hasbeen achieved in lenses in which the asymmetry ratios fall within thefollowing ranges:

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention. In particular, it should be understood that throughout thisdescription it is assumed that light is traveling from left to rightthrough the lenses illustrated in FIGS. 1 and 4. Because the object andimage distances are substantially the same, the object and imagepositions may be interchanged. With the object on the right, light wouldpass from right to left through the lenses as illustrated in FIGS. 1 and4 and the entrance pupil would be physically located near the left endof the lens while the exit pupil would be located near the center of thelens. However, the thickness and air space ratios described hereinbeforewill lie within the ranges already disclosed, as these ratios are notdependent upon the direction of passage of the light through the lens,but upon the relative size and spacing of the lens elements.

I claim I. A lens comprising six air spaced elements, said elementsbeing, in consecutive order, a first positive biconvex element, a firstpositive meniscus element con- I ment, wherein:

the ratio of the thickness of said second negative elecave away fromsaid first biconvex element, a first negative element, a second negativeelement, a second positive meniscus element concave toward said secondnegative element and a second positive biconvex element to said firstnegative element has a value between 2.0 and 5.0;

the ratio of the air space separating said second negative and saidsecond positive meniscus elements to the air space separating said firstnegative and said first positive meniscus elements has a value between1.0 and 9.0; and the ratio of the air space separating said secondpositive elements to the air space separating said first positiveelements has a value between 1.0 and 9.0. 2. A lens comprising, fromfront to rear, a positive biconvex element, a positive meniscus elementconcave I to the rear, a negative element, an aperture stop, a negativeelement, a positive meniscus element concave to the front and a positivebiconvex element, said lens having an equivalent focal length of 100millimeters when constructed according to the parameters in thefollowing table:

Radius Thickness or Element N V mm Separation mm S, 1.060 R5 m 3 1.6258835.7 T, 3.523

S, 6.643 R-, 39.814 4 1.69895 30.1 T, 14.517

. S, 8.189 R, 112.62 5 1.73400 51.3 T, 3.065

S, 8.474 R 176.61 6 1.71300 53.9 T,=4.530

wherein, from front to rear, the lens elements are numbered from 1-6,the corresponding indices of refraction, N, and Abbe numbers, V, are forthe 0.5876 micron Helium line of the spectrum, the radii are numberedfrom R, to R the thicknesses are numbered from T, to T and the airspaces are numbered from S, to 8,.

3. A lens comprising, from front to rear, a positive biconvex element, apositive meniscus element concave to the rear, a negative element, anaperture stop, a negative element, a positive meniscus element concaveto the front and a positive biconvex element, said lens having anequivalent focal length of millimeters when constructed according to theparameters in the following table:

wherein, from front to rear, the lens elements are numbered from l-6,the corresponding indices of refraction, N, and Abbe numbers, V, are forthe 0.5876 micron Helium line of the spectrum, the radii are numberedfrom R to R the thicknesses are numbered from T to T and the air spacesare numbered from S, to S 4. A lens comprising from front to rear, apositive biconvex element, a positive meniscus element concave to therear, a negative element, an aperture stop, a negative element, apositive meniscus element concave to the front and a positive biconvexelement, said lens having an equivalent focal length of 100 millimeterswhen constructed according to the parameters in the following table:

Radius Thickness or I Element N V mm Separation mm S, 1.975 R, 41.102 21.73400 51.3 T,=3.080'

S, 2.740 R, 1505.0 3 1.62588 35.7 T, 6.003

S; 7.41 1 R 38.719 4 1.69895 30.1 T, 12.888

S, 3.677 R, 113.07 5 1.73400 51.3 T, 2.823

S, 16.881 R 196.55 6 1.71300 53.8 T, 4.394

numbered from S to S

1. A lens comprising six air spaced elements, said elements being, inconsecutive order, a first positive biconvex element, a first positivemeniscus element concave away from said first biconvex element, a firstnegative element, a second negative element, a second positive meniscuselement concave toward said second negative element and a secondpositive biconvex element, wherein: the ratio of the thickness of saidsecond negative element to said first negative element has a valuebetween 2.0 and 5.0; the ratio of the air space separating said secondnegative and said second positive meniscus elements to the air spaceseparating said first negative and said first positive meniscus elementshas a value between 1.0 and 9.0; and the ratio of the air spaceseparating said second positive elements to the air space separatingsaid first positive elements has a value between 1.0 and 9.0.
 2. A lenscomprising, from front to rear, a positive biconvex element, a positivemeniscus element concave to the rear, a negative element, an aperturestop, a negative element, a positive meniscus element concave to thefront and a positive biconvex element, said lens having an equivalentfocal length of 100 millimeters when constructed according to theparameters in the following table: Radius Thickness or Element N V mmSeparation mm R1 51.804 1 1.78831 47.4 T1 5.050 R2 -270.13 S1 6.714 R338.733 2 1.73400 51.3 T2 3.158 R4 86.041 S2 1.060 R5 Infinity 3 1.6258835.7 T3 3.523 R6 28.146 S3 6.643 R7 -39.814 4 1.69895 30.1 T4 14.517 R8156.48 S4 8.189 R9 -112.62 5 1.73400 51.3 T5 3.065 R10 -53.643 S5 8.474R11 176.61 6 1.71300 53.9 T6 4.530 R12 -91.830 wherein, from front torear, the lens elements are numbered from 1-6, the corresponding indicesof refraction, N, and Abbe numbers, V, are for the 0.5876 micron Heliumline of the spectrum, the radii are numbered from R1 to R12, thethicknesses are numbered from T1 to T6 and the air spaces are numberedfrom S1 to S5.
 3. A lens comprising, from front to rear, a positivebiconvex element, a positive meniscus element concave to the rear, anegative element, an aperture stop, a negative element, a positivemeniscus element concave to the front and a positive biconvex element,said lens having an equivalent focal length of 100 millimeters whenconstructed according to the parameters in the following table: RadiusThickness or Element N V mm Separation mm R1 52.093 1 1.78821 47.5 T15.080 R2 -266.30 S1 6.862 R3 38.752 2 1.73252 51.7 T2 3.135 R4 85.900 S20.983 R5 Infinity 3 1.62552 35.8 T3 3.619 R6 28.267 S3 6.483 R7 -39.8344 1.69864 30.1 T4 14.332 R8 158.81 S4 8.495 R9 -113.08 5 1.73252 51.7 T53.053 R10 -53.670 S5 8.202 R11 177.80 6 1.71324 54.0 T6 4.515 R12-92.566 wherein, from front to rear, the lens elements are numbered from1-6, the corresponding indices of refraction, N, and Abbe numbers, V,are for the 0.5876 micron Helium line of the spectrum, the radii arenumbered from R1 to R12, the thicknesses are numbered from T1 to T6 andthe air spaces are numbered from S1 to S5.
 4. A lens comprising fromfront to rear, a positive biconvex element, a positive meniscus elementconcave to the rear, a negative element, an aperture stop, a negativeelement, a positive meniscus element concave to the front and a positivebiconvex element, said lens having an equivalent focal length of 100millimeters when constructed according to the parameters in thefollowing table: Radius Thickness or Element N V mm Separation mm R153.240 1 1.78831 47.4 T1 5.130 R2 -248.90 S1 1.975 R3 41.102 2 1.7340051.3 T2 3.080 R4 84.269 S2 2.740 R5 1505.0 3 1.62588 35.7 T3 6.003 R627.657 S3 7.411 R7 -38.719 4 1.69895 30.1 T4 12.888 R8 202.20 S4 3.677R9 -113.075 1.73400 51.3 T5 2.823 R10 -48.741 S5 16.881 R11 196.55 61.71300 53.8 T6 4.394 R12 -92.355 wherein, from front to rear, the lenselements are numbered from 1-6, the corresponding indices of refraction,N, and Abbe numbers, V, are for the 0.5876 micron Helium line of thespectrum, the radii are numbered from R1 to R12, the thicknesses arenumbered from T1 to T6 and the air spaces are numbered from S1 to S5.